Chemical synthesis of Nd2Fe14B/Fe-Co nanocomposite with high magnetic energy product

Abstract

Nd₂Fe₁₄B is one of the most popular permanent magnets (PMs) possessing the best energy product (BH)_max among the common PM materials. However, exchange-coupled nanocomposite magnets fabricated by embedding nanostructures of soft-phase magnetic materials into a hard-phase magnetic matrix manifest higher remanence and a higher energy product. Here we present the fabrication of exchange coupled Nd₂Fe₁₄B/Fe-Co magnetic nanocomposites using gel-combustion and diffusion-reduction processes. Pre-fabricated CoFe₂O₄ nanoparticles (NPs) of ∼5 nm diameter were incorporated into a Nd-Fe-B oxide matrix during its synthesis by gel-combustion. The obtained mixed oxide was further processed with oxidative annealing at 800 °C for 2 h and reductive annealing at 900 °C for 2 h to form a Nd₂Fe₁₄B/Fe-Co nanocomposite. Nanocomposites with different mol% of soft-phase were prepared and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and physical property measurement system (PPMS) to study their crystalline phase, morphology and magnetic behavior. Addition of 7.7 mol% of soft-phase was found to be optimum, producing a coercivity (H _c) of 5.6 kOe and remanence (M _r) of 54 emu g^-1 in the nanocomposite.

Fig. 1. Schematic presentation of the sequential steps of fabricating Nd₂Fe₁₄B/Co–Fe exchange-coupled nanocomposite.

Fig. 2. Flow chart of the processing steps used for the synthesis of exchange coupled Nd–Fe–B/Fe–Co nanocomposites.

Fig. 3. (a) A typical TEM image, (b) particle size distribution, and (c) XRD pattern of the fabricated cobalt ferrite NPs. Gaussian fit to the size distribution histogram revealed ∼5 nm average size of the particles. A typical optical image of sol–gel combustion synthesis procedure shown in (d).

Fig. 4. SEM image (a), TEM image (b), HAADF image (c). HRTEM (d) of NdFeB-oxide (e) XRD patterns of the oxide composites containing different amounts of soft-phase oxide, and typical TEM images of Nd–Fe–B oxide, 92.3 mol% Nd–Fe–B oxide/7.7 mol% Fe–Co oxide, 90.5 mol% Nd–Fe–B oxide/9.5 mol% Fe–Co oxide, 73.1 mol% Nd–Fe–B/26.9 mol% Fe–Co oxide, 57.7 mol% Nd–Fe–B/42.3 mol% Fe–Co oxide composites. All the samples were air-annealed at 800 °C for 2 h.

Fig. 5. EDS elemental mappings images of the (a) Nd–Fe–B oxide (with no Fe–Co oxide), (b) 92.3% Nd–Fe–B oxide/7.7% Fe–Co oxide, and (c) 73.1% Nd–Fe–B oxide/26.9% Fe–Co oxide samples after air-annealing at 800 °C for 2 h (scale bar 500 nm).

Fig. 6. (a) XRD patterns of the exchange-coupled (100 – x)% Nd–Fe–B/x% Fe–Co magnetic nanocomposites obtained after reduction–diffusion of corresponding oxide mixtures, (b) HRTEM image of NdFeB nanoparticles, (c) HRTEM image of reduced NdFeB/FeCo composite 7.7%.

Fig. 7. (a) Room temperature M − H curves of NdFeB phase and FeCo phase, (b) room temperature magnetization (M − H) curves, (c) coercivity, (d) remanence and (e) magnetic saturation for the exchange-coupled (100 − x)% Nd–Fe–B/x% Fe–Co nanocomposites containing different mol% (x) of Fe–Co nanoparticles.